As defined in MIL-STD-753C, Passivation is the final treatment/cleaning process used to remove iron from the surface of corrosion resistant steel parts such that a more uniform formation of a passive surface is obtained thus enhancing corrosion resistance.
Stainless steel is different from other metals in that as you get closer to the surface the composition of the metal actually changes. In the passivation process, free iron is removed from the surface into solution, leaving behind a higher chromium level. A good chrome to iron ratio is usually considered to be 1.5 to 1 or higher.
Passivation achieves 3 things:
1). Removal of surface contamination (metal oxides, inclusions, fabrication debris and bio-contamination).
2). Improvement or formation of the passive (inert) layer for increased corrosion resistance.
3). Reduce corrosion vulnerability and lower product contamination potential.
Passivation of course also extends system maintenance intervals as well as meets cGMP/BPE surface quality requirements